This invention relates to a process for separating nitrogen gas by the pressure swing adsorption system (hereinafter referred to as "PSA system"). More particularly, it relates to a process for separating nitrogen gas of extremely high purity from a raw material gas mixture predominantly composed of nitrogen and oxygen, for example air, by means of an adsorber (adsorption bed) packed with a carbon (carbonaceous) molecular sieve (hereinafter referred to as "CMS") as an adsorbent.
The process for separating nitrogen gas from such a mixed gas by the PSA system by utilizing the difference in rate of adsorption on CMS between oxygen gas and nitrogen gas is already in practical use. The equilibrium adsorption of oxygen gas and that of nitrogen gas on CMS are almost equal to each other under the same temperature and pressure conditions. Just after initiation of adsorption, however, these gases show a remarkable difference in rate of adsorption. Thus, oxygen gas and water vapor are each adsorbed on CMS almost to the equilibrium adsorption in about 60 seconds, while nitrogen gas and argon are adsorbed little. The separation of nitrogen gas by the PSA system utilizes this characteristic.
The purity of nitrogen gas obtainable in practical processes using the PSA system is generally said to be at most 99.0% in the case of atmospheric pressure regeneration and at most 99.9% in the case of reduced pressure regeneration. It is regarded as difficult to separate nitrogen gas with a purity of 99.9% or higher in an economic manner by said system. It is a matter of course that any apparatus in which nitrogen gas having a high purity of 99.99% or more can be obtained is not known as yet.
Therefore, in cases where nitrogen having a higher purity is required, it is a common practice to combinedly use a purification apparatus for converting the remaining 0.1 to 1.0% by volume of oxygen to water by reaction with hydrogen gas, ammonia or the like in the presence of a catalyst and removing the resulting water, or for removing such oxygen by contacting with a reducing metal. In operating such purification apparatus, however, a highly dangerous gas is used for reaction or regeneration, as mentioned above. In addition, there remain many problems, such as contamination with unreacted gases, and marked increases in cost of equipment and cost of operation.
On the other hand, the prior art processes for producing high-purity nitrogen, typically the low temperature separation process, generally require a large-scale equipment, hence a large amount of investment. Accordingly, the site for plant construction is limited, and the cost of transportation to sites of consumption increases. For these reasons, the cost of nitrogen gas becomes very high.
As mentioned above, the separation of nitrogen gas by the PSA system is advantageous in that nitrogen gas can be readily obtained at low cost and by relatively simple mechanisms but the product nitrogen gas has a limited range of applications because of its low purity. The purity may be increased by addition of a purification apparatus, but with many demerits, as mentioned above.
Accordingly, a gas supply source which can produce high-purity nitrogen gas while retaining the advantageous features of the PSA system has been highly demanded in the fields of chemical industry, food industry, semiconductor industry and metal heat treatment industry, among others.